1. Field of the Invention
The present invention relates to an apparatus for making a master by perforating a stencil with a thermal head or similar heating device in accordance with image data, and a stencil unit forming part of such an apparatus.
2. Discussion of the Background
A digital thermosensitive stencil printer is a simple and convenient printing implementation known in the art. This type of printer has a thermal head on which minute heating elements are arranged in an array. While the head is held in contact with a stencil being conveyed, a current is selectively fed to the heating elements pulses in accordance with image data. As a result, the stencil is selectively perforated by heat to turn out a master. Subsequently, ink is transferred to a sheet via the perforations of the master to print out the image data.
The stencil for use with the printer of the type described may be implemented as a roll, as disclosed in Japanese Patent Laid-Open Publication No. 6-32041 by way of example. The roll is stored in an accommodating section formed in the printer, i.e., in a master making apparatus thereof. Specifically, the operator takes out a fresh stencil roll from a package, removes a tape or similar retaining member from the leading edge portion of the stencil, and then sets the roll in the accommodating section. Subsequently, the operator pulls out the leading edge portion of the stencil from the accommodating section, causes the master making apparatus to nip it, causes a conveyor to convey the stencil a predetermined amount, causes a cutter to cut off the leading edge, and then removes the cut piece of the stencil from the printer by hand.
However, the operation for mounting the stencil roll to the accommodating section of the printer described above is extremely troublesome. Moreover, since the stencil is apt to crease and bring about defective printings if handled carelessly, the mounting operation has to be performed only by an experienced person.
The stencil in the form of a roll has a laminate structure consisting of a porous substrate permeable to ink and a film of thermoplastic resin. The porous substrate is made of ordinary Japanese paper, synthetic fibers, or a mixture thereof. The problem with such a substrate is that the permeability to ink is not constant due to irregularities in the diameter, length, thickness and density of fibers constituting the substrate. Specifically, the permeability of ink is irregular when the fibers aggregate at a number of perforated portions of the film or when comparatively thick fibers traverse the perforations of the film. Further, the substrate and the thermoplastic film are joined together by adhesive. Should the distribution of the adhesive be irregular, the permeation of ink would be obstructed at the perforations of the film where a great amount of adhesive exists. Then, even if the film is successfully perforated by heat in accordance with image data, the portions of the substrate where the permeability is low cause the resulting image to be locally lost. This is particularly true when a solid image having a substantial area is printed.
Japanese Patent Laid-Open Publication No. 63-30295 discloses an approach to eliminate an occurrence that the stencil melts and adheres to the heating elements of the thermal head to result in defective perforations. The approach consists in providing a stencil storing device having a stencil guide portion, and storing a stencil roll in such a device. Even this kind of scheme is not satisfactory since the operator has to cause, by hand, the guide portion to nip the leading edge of the stencil pulled out from the roll, often creasing the stencil.
Further, Japanese Patent Laid-Open Publication No. 60-87094 teaches a stencil having a porous substrate which is implemented by a regular arrangement of fibers having the same diameter in place of Japanese paper. This kind of substrate is adhered to a thermoplastic resin film. However, the problem with this scheme is that even when a new master just perforated is wound round a drum of a printer, ink cannot reach the thermoplastic film unless it sufficiently infiltrates into the porous substrate. Hence, at the beginning of printing, a few sheets should be wasted until an acceptable image is attained. The used master is removed from the drum, collected in a preselected discharge section, and then discarded. Since the used master is discarded with a great amount of ink contained in the substrate, the running cost of ink increases.
In light of the above, there has been proposed a stencil having a thin porous substrate, and a stencil implemented substantially only by a thermoplastic resin film, i.e., lacking a porous substrate. However, while the conventional laminate type stencil is 40 .mu.m to 50 .mu.m thick, the stencil implemented substantially only by a thermoplastic resin film is as thin as 2 .mu.m to 8 .mu.m and, therefore, not sufficient in strength. Such a thin stencil is apt to adhere to the operator's hand, guide and so forth due to static electricity when it is set on a printer, again bringing about the crease problem. In addition, dust, including paper dust, deposited on the film would prevent it from being surely perforated by heat.